So, basically the paper was written and submitted as student. But PhD is over, so not anymore a student. Is there any way to still attend it as a student. The fee for a regular is several times that of a student.
You don't "attend it as a student" --- you just attend it because you were already invited and then you can give your audience an introduction where you update them on what you're doing now. As to the fee, you should seek clarification from the conference organisers. If you now have a professional academic position then it is likely the conference organisers will want to charge the full fee, and your university ought to be able to pay it for you. If not, you might be able to haggle over what fee you need to pay to go.